The present invention relates to electrostatic atomizers.
As described in U.S. Pat. No. 4,255,777, the disclosure of which is incorporated by reference herein, a fluid can be atomized by injecting an electrically charge into it. In certain embodiments taught in the ""777 patent, the fluid passes through a housing and out of a discharge orifice defined by a wall of the housing. An electron emitter electrode, also referred to as a charge injection electrode, is disposed immediately upstream of the discharge orifice. Typically, the wall defining the orifice itself is a metallic body and serves as a second electrode. The second electrode which is maintained at a different electrical potential from the charge injection electrode or emitter. Under these conditions, electric charges leave one of the electrodes and move towards the other electrode through the fluid For example, where the emitter is maintained at a negative potential with respect to the second or aperture electrode, electrons leave the emitter electrode and move towards the second electrode through the flowing fluid. Because the charges have a finite, limited velocity within the flowing fluid, some or all of the charge is carried out through the orifice with the flowing fluid before such charge reaches the second electrode. The stream of fluid passing out of the orifice thus carries a net charge. Because the fluid has a net charge, the various portions of the fluid repel one another. Such repulsion causes the fluid to break apart or atomize The charges are ultimately discharged by a third, ground electrode outside of the orifice.
Other electrostatic atomization systems described in certain preferred embodiments of U.S. Pat. Nos. 5,093,602 and 5,378,957, the disclosures of which are hereby incorporated by reference herein, utilize electron beams to introduce charge into the fluid. Systems of this type have a small electron gun mounted adjacent to discharge orifice. Typically, the electron gun incorporates a housing having an interior space maintained under vacuum. An electron-transmissive window is provided over on opening in the electron-gun housing. A cathode and accelerating electrodes within the electron gun form an electron beam which is directed through the window into the fluid as the fluid passes into and through the orifice. Here again, a net charge is introduced into the fluid and the fluid is atomized by mutual repulsion between charged portions of the fluid.
Electrostatic atomization systems as discussed above offer numerous advantages over conventional atomization systems. In particular, the degree of atomization is controlled by the amount of charge introduced into the fluid. The preferred systems described in the aforementioned patents can apply substantial net charge and can provide very effective atomization. These systems do not depend upon mechanical action for atomization. Thus, it is possible to dispense with many of the elements commonly found in mechanical atomization systems. For example, there is no need to force the fluid through a fine orifice at a high flow rate to induce atomization by shear, and no need to supply high-velocity jets of compressed gas to induce atomization. The system can operate at low fluid pressures and with any desired flow rate. These features facilitate construction of simple, light weight atomization systems. Moreover, because droplet size is strongly controlled by the amount of charge injected, the system can achieve the desired degree of atomization despite variations in fluid flow rate and fluid properties such as viscosity. The systems can operate with small amounts of electrical power. Systems of these type can be used to atomize numerous different materials. However, one significant application for such systems has been in atomization of liquid fuels such as fuel oil, diesel oil, kerosene and jet engine fuel in engine and combustion applications. For example, systems of this type can be used in place of conventional fuel injectors in diesel engines and in gas turbine engines.
Because electrostatic atomization of this type can provide effective atomization even with very low flow rates, fuel can be atomized at flow rates appropriate to provide a flame having few watts to a few hundred watts of heat output. As described in co-pending, commonly assigned U.S. application Ser. No. 09/237,583, filed Jan. 26, 1999, the disclosure of which is hereby incorporated by reference herein, a small burner which provides such a flame can be used, for example, as the heating element in a small, simple cooking stove for use by an individual soldier or camper. As described in greater detail in the ""583 application, such a low flow atomizer typically includes a small orifice at the atomization nozzle for regulating the fluid flow. The nozzle may be of variable size to provide variable flow rate.
Despite these and other improvements in electrostatic atomization, still further improvement would be desirable. Electrostatic atomization systems using small orifices, and especially those using orifices less than about 100 xcexcm in diameter, can become clogged with sooty particulates. Although the present invention is not limited by any theory of operation, it is believed that this soot arises from some side effects of the electric fields applied to the fluid such as the field applied between the emitter electrode and the second or counter electrode. Thus, it is believed that phenomena associated with injection of charge into the fluid to be atomized cause chemical reactions to occur in the vicinity of the charge injection electrode or electron gun. Such reactions may cause polymerization of the fluid, particularly where the fluid is a organic liquid such as a liquid fuel. Regardless of the cause however, sooty particles tend to form inside the atomization device. These particles generally do not pose a problem in systems using relatively large discharge orifices, such as those above about 100 microns in diameter and particularly above about 500 mm in diameter. However, small apertures, particularly those below about 100 microns in diameter, can become clogged in as little as an hour of operation. Reducing the applied voltage can increase the time required for clogs to form. However, this does not provide a complete solution to the problem and limits the capability of the system. Thus, a better solution to the clogging problem would be desirable.
Moreover, it would be desirable to provide arrays of electrostatic spray nozzles. For example, in a small burner, it would be desirable to provide multiple plumes of atomized liquid fuel to provide multiple, small flames. In particular, it would be desirable to provide such an array in a form which can be manufactured readily at low cost.
The present invention addresses these needs.
One aspect of the present invention provides an electrostatic atomizer incorporating a body defining an interior space, and an exterior surface. The body also defines a fluid entry port communicating with the interior space remote from the orifice. A charge injection structure is disposed within the interior space in the vicinity of the orifice. The charge injection structure may be an emitting electrode or an electron gun as discussed above. Desirably, a counter electrode is disposed in the vicinity of the orifice. According to this aspect of the present invention, a dielectric structure is disposed between the counter electrode and the interior space, so that the counter electrode is electrically insulated from the interior space. For example, the body may incorporate a dielectric material and the dielectric material of the body may serve as the dielectric structure which insulates the counter electrode from the interior space. The counter electrode desirably overlies the exterior surface of the body on the vicinity of the orifice. The dielectric material of the body may define the orifice and the exterior surface and the counter electrode may be in the form of an electrically conductive coating on the exterior surface of the body.
Atomizers according to this aspect of the invention desirably are substantially devoid of electrically conductive surfaces exposed to the interior space other than the electrically conductive surfaces of the charge injection structure itself which are at the same electrical potential as the charge injection structure. Stated another way, the atomizer desirably does not apply electric fields in excess of about 1000 V/mm between electrically conductive surfaces exposed to the interior space, and most desirably does not apply any electric fields between electrically conductive surfaces exposed to the interior space. There may be substantial electric fields between the charge injection structure and a counter electrode outside of the body.
Atomizers according to this aspect of the invention incorporate the discovery that formation of large soot particles which cause plugging within the atomizer can be suppressed by insulating electrically conductive structures such as the second or counter electrode from the flowing fluid. Despite the dielectric exposed between the electrode and the flowing fluid, the electrode still performs the required function for atomization. For example, in a triode-type device, the electric field between the emitter electrode and the second electrode is imposed through the fluid and through the dielectric structure. Atomization proceeds substantially in the same way as where the second electrode is on contact with the flowing fluid. However, large soot particles do not form inside the atomizer and do not clog the discharge orifice, even if the orifice is of small diameter. Here again, although the present invention is not limited by any theory of operation, it is believed that by limiting or eliminating electrical fields between conductive elements, electrical currents flowing between conductive elements inside the interior space are reduced, and that this in turn reduces the tendency of sooty material formed during charge injection to agglomerate or settle on surfaces of the apparatus.
A further aspect of the present invention provides an atomizer incorporating a body having one or more interior spaces and a first wall bounding the one or more interior spaces. The first wall defines an exterior surface of the body and a plurality of orifices extending between the exterior surface and the one or more interior spaces. The body also has one or more fluid inlets communicating with the one or more interior spaces remote from the orifices. A plurality of charge injection electrodes desirably are mounted in the one or more interior spaces and are disposed adjacent the orifices. One or more counter electrodes are also disposed adjacent the orifices. Desirably, the one or more counter electrodes are disposed on the exterior surface of the first wall, and the first wall includes a dielectric structure facing the interior space or spaces to insulate the one or more counter electrodes from the interior space or spaces and provide the advantages discussed above. The body desirably includes a second wall extending generally parallel to the first wall and internal structure extending between the first and second walls. The one or more interior spaces are disposed between the first and second wall. The charge injection electrodes may be mounted to the second wall in alignment with the orifices in the first wall. Atomizers of this type may include large numbers or orifices and may include small orifices as, for example, orifices less than about 200 microns meters in diameter. The orifices may be spaced apart from one another by less than about 500 micrometers. In a particularly preferred arrangement, the orifices are less than about 50 microns in diameter and are spaced apart from one another by less than about 125 micrometers. As further discussed below, structures of this type can be formed by microstructure fabrication techniques similar to those commonly employed to a manufacture semiconductor chips and the like. As also discussed below, the optimum spacing between the charge injection electrodes and the discharge orifices is directly related to orifice size. The use of numerous small orifices instead of a single larger orifice thus permits the use of smaller spacing, which in turn allows operation at lower voltages. Operation at lower voltages permits the use of simpler, more economical power supplies. Preferred atomization devices according to this aspect of the invention can be made to provide essentially any desired flow rate at any specified fluid inlet pressure.